Numerical correction methods of neutron damage in position-sensitive HPGe detectors

The Advanced GAmma Tracking Array (AGATA) is a major γ-ray spectrometer mainly for nuclear structure studies. AGATA is based on the novel technique of γ-ray tracking in electrically segmented high-purity Ge crystals. The array is currently employed at the Grand Accelerateur National d'Ions Lourds (GANIL, France) in stable-beam experiments with high count rates. Fast neutrons are emitted after almost any nuclear reaction with projectile energies above the Coulomb barrier. These neutrons generate crystal defects by dislocating Ge atoms. Dislocations act as hole traps within the HPGe material, causing a reduced charge collection efficiency, observed as a left tailing in the energy-peak shapes. The crystals can recover from neutron damage by annealing. However, for practical reasons, this treatment cannot be applied after every experiment. Recently, two software-based methods were developed employing pulse-shape analysis (PSA) to minimize the trapping effects between consecutive scheduled annealings of the HPGe crystals. Both approaches employ the position sensitivity of the HPGe detectors and determine the neutron trapping contribution for charge collection after individual γ-ray interactions. Energy resolution and line shape of neutron-damaged detectors are improved dramatically. Results and perspectives on neutron-damage corrections are presented.